1. Technical Field of the Invention
The present invention relates to a micromachining process that is compatible with dry-etchable sacrificial layers, thick mono-crystalline suspended layers, on-chip electronics on the substrate and, in particular, to a interconnect between the thick mono-crystalline suspended layers and the microelectronics on the substrate.
2. Description of the Related Art
A micro-electro-mechanical system (MEMS) is a device that transforms mechanical activity into electrical signals and vice versa. A key element in MEMS is a variable capacitor formed between a stationary electrode and a movable electrode attached to a suspended proof mass. The movable electrode deflects in response to acceleration or Coriolis force exerted on the proof mass, or electrical voltage applied between the stationary and the movable electrodes. The amount of deflection can be sensed from changes in capacitance from the changes in the gap between the two electrodes due to deflection. The amount of deflection can also be sensed with piezoelectric or piezo-resistive layers formed on the microstructure. One example is an accelerometer that comprises a rectangular membrane, one side of which is fixed to a carrier while the other is suspended, and a means for detecting the movement of the membrane under the effect of acceleration. This constitutes a sensor, which senses acceleration force. Conversely, an electrical signal applied to the capacitive plates or piezoelectric layers can result in deflection in the movable elements. This generates a force internally that can be used as an engine to drive the MEMS device. MEMS devices may comprise both drive elements and sensing elements to perform specific functions. One such example is a MEMS gyroscope fabricated with micromachining techniques.
There are two major classifications of methods for making thin suspended microstructures: (1) Bulk micro-machining, where the transducers are primarily shaped by etching a thick mono-crystalline silicon substrate; and (2) Surface micro-machining, where transducers are constructed entirely from thin films deposited on respective sacrificial layers. Mono-crystalline silicon used in bulk micro-machining has two major benefits. The first benefit is the silicon is almost a perfect mechanical material. It's stronger than steel by weight; does not show mechanical hysteresis and is highly sensitive to stress. This technology requires a deep silicon etch to remove the bulk of the material to form the microstructures. This is normally done by anisotropic wet etch where both dimension control and contamination control is a major challenge. An important feature of bulk micro-machining is the microstructure is often bonded to another wafer for packaging. The bonding techniques include anodic bonding, metallic seals, and low-temperature glass bonding. Polymer bonding is attractive because it is widely used in attaching semiconductor dies to substrates for packaging, but it is rarely used in micro-machining as most of these applications require very thin bondlines. The application of a thin layer of polymer adhesive requires thinning with solvent that is incompatible with most micro-machining techniques.
Surface micro-machined MEMS devices are constructed entirely from thin films deposited on a sacrificial layer. These devices allow for monolithic integration with silicon processors using the standard silicon process technology commonly known to individuals skilled in the art. The sacrificial layer is either made of a polymer such as photoresist or an inpolymer substance such as silicon oxide. Photoresist can be dry etched in oxygen plasma but can not withstand high temperature anneal like silicon dioxide can. Chemical-vapor deposited polysilicon film is used in integrated accelerometers and many other MEMS devices. Unfortunately, it must be annealed at a high temperature (˜1000° C.) to reduce stress, or its suspended structure will curl after the sacrificial layer is removed from underneath. The sacrificial layer is made of silicon dioxide whose removal is by wet etch in HF acid. Surface tension of the aqueous HF acid solution exerts forces on the suspended microstructure, which pulls the microstructure into contact with the substrate and causes them to stick together. To separate them without causing damage is difficult because the combination of adhesive forces and electrostatic forces is large compared to the strength of the thin films. Another drawback is that the metal interconnect in the integrated control circuits cannot withstand the high temperature anneal. Therefore, it must be formed after the polysilicon suspended structure is formed. Protecting the suspended structure during the interconnect process and the wet etch is a complex matter and entails the usage of costly state-of-the-art fabrication facilities.
Surface micro-machining that does not require high temperature anneal has distinct advantages because dry etch can be used for removing the sacrificial layer and the microstructure can be fabricated on finished integrated circuits. This avoids the sticking associated with the wet etch process and the expensive equipment thereby necessitated. However, the intrinsic stress and hysteresis in the deposited film limits its thickness to a few thousand angstroms or the films can curl and change after stressing. This makes the technique not suitable for devices such as accelerometers that require larger proof mass.
Therefore, it is highly desirable to combine the merits of both bulk and surface micro-machining techniques—such as MEMS devices employing suspended structures made from mono-crystalline silicon by a surface micro-machining method. One example is surface micro-machined accelerometers made with silicon-on-insulator (SOI) wafers, wherein thick mono-crystalline silicon is bonded to another silicon wafer with a silicon oxide insulator in between. The suspended structure can be made much thicker than mono-crystalline for increased proof mass. However, SOI wafers are much more expensive than the regular wafers and its production employs a high temperature process that is not compatible with integrated circuits and dry-etchable sacrificial layers. It would be of great advantage if the sacrificial layer were dry-etchable.
Another issue related to non-dry-etchable sacrificial layers relates to packaging, where they must be wet etched and movable elements released prior to packaging. Extreme care must be taken to attach these MEMS devices during packaging.
U.S. Pat. No. 6,060,336 describes a method to fabricate MEMS devices that combines the merits of both bulk micromachining and surface micromachining techniques. It employs mono-crystalline silicon for making suspended structures from and polymer adhesive as the dry-etchable sacrificial layer, spacer and pillar for supporting the suspended structures. The method solves most of the aforementioned issues in MEMS manufacturing and cuts the cost of packaging where release of the suspended structures can be done after the devices are mounted in the packages.
One potential drawback for MEMS devices that employ mono-crystalline suspended structures such as that manufactured with the method provided in U.S. Pat. No. 6,060,336, is that electrical interconnection between the suspended structures and the microelectronics is with bond wires, which require bonding pads and area. Thus, the number of bond wires interconnection is limited.
Another potential drawback for such MEMS devices which employ polymer adhesive pillars to bond and support mono-crystalline suspended structures with the substrate is the polymer adhesive pillars may outgas and deteriorate vacuum packaging. Thus there is a need to employ metal interconnects that double as pillars, so that the polymer adhesive layers can be removed totally.
All MEMS inertial sensors such as gyroscopes and accelerometers operate on the principle of proof mass suspended by flexures anchored to the substrate. The flexures serve as the flexible suspension between the proof mass and the substrate, making the mass free to move in response to external forces or acceleration. The movement can be measured with a built-in variable capacitor or piezoelectric sense elements. In the case of MEMS gyroscopes, the proof mass is driven into vibratory motion in the x-direction by an external sinusoidal force with the drive. If the gyroscope is subjected to an angular rotation in the z-direction, the Coriolis force is induced in the y-direction. The resulting oscillation amplitude in the sense direction is proportional to the Coriolis force, and thus to the angular velocity to be measured. These MEMS gyroscopes are very small in size to satisfy yield and cost considerations, thus signal response is very weak, due in part to the smallness of the proof mass and sense capacitors, making it difficult to achieve the high accuracy needed for gyroscopes used in navigation applications. Thus, it is highly desirable to increase both the area and thickness of the suspended layer in these devices to increase the signal strength; likewise for MEMS accelerometers. Both the proof mass and sense capacitors must be large enough to archive desired accuracy. In addition, there is a need to have integrated electronics on the same chip to eliminate long metal leads that are used to connect the MEMS with off-chip electronics and can pick up excess noise. Furthermore, there is a need to use thermally matched substrates as a base for the suspended structure to anchor to for improved stability during operation, as thermal stress may change the drive and sense resonant frequencies and induce errors.
Unfortunately, surface micromachining techniques base on LPCVD polysilicon layers have very limited layer thickness (˜5 μm) and suffer from residue stress from the deposition process. The bulk micromachining process uses mono-crystalline silicon to form the suspended structures, such as the proof mass, and does not suffer from residue stress and thickness limitations. But the substrate that the suspended structure is bonded to is normally made of glass. Therefore, there is a significant amount of mismatch in thermal expansion coefficients between the two that cause errors from fluctuations in operating temperature. Furthermore, readout and control electronics cannot be integrated on the same chip. This substantially increases complexity of the gyroscope subsystem and thus cost, size, and power consumption. Thickness of the silicon layer is larger but still limited, to ˜10 μm, in a tuning-fork MEMS gyroscope fabricated using this technique.
Hence, there is a strong need in the industry to overcome the aforementioned shortcomings of the present art.